Air conditioning systems are routinely employed within automobiles and other vehicles for creating comfortable conditions within the passenger compartment for the vehicle occupants. At outside temperatures above about 70.degree. F., it is difficult to maintain a comfortable passenger compartment temperature without first cooling the air that is being blown into the passenger compartment.
Typically, cooling of the air is accomplished by first compressing an appropriate refrigerant, such as the fluorocarbon known as freon or another alternative refrigerant. Within an automobile, the engine-driven compressor compresses the vaporized refrigerant, thereby significantly raising the temperature of the refrigerant. The refrigerant then flows into a condenser where it is cooled and returned to its liquid state; thus, the heat added to the refrigerant in the compressor is transferred out of the system by means of the condenser. The cooled liquid refrigerant is then sprayed through an expansion valve into an evaporator where it is again vaporized. The required heat of vaporization is drawn from the incoming air. Any excess humidity contained within the incoming air is removed as condensation on the evaporator, therefore also drying the incoming air. The cooled, dry air then enters the passenger compartment of the vehicle.
So as to efficiently maximize the amount of surface area available for transferring heat out of the refrigerant in the condenser by the incoming air, the design of the condenser unit is typically a tube-and-center type heat exchanger containing a multitude of tubes interspaced by high surface area fins centered between the tubes. The evaporator unit is often formed of a similar tube-and-center type design.
Conventionally, the condenser has been constructed by inserting the tubes within oppositely disposed headers and then centering the fins between the tubes. Inlet and outlet means fluidically attached to one or more refrigerant reservoirs and in fluidic communication with the tubes are also assembled to the tube and header assembly. The entire assembly is then brazed together.
A problem arises in that the condenser contains a multitude of internal tubes which all must be brazed to the headers during a single brazing operation. There are practically scores of these brazements which must be formed concurrently. Generally, this is accomplished by employing an aluminum alloy brazing stock material for formation of the matched headers. The aluminum alloy brazing stock material consists, for example, of an appropriate aluminum alloy core which has been clad on at least one side with an aluminum-based brazing alloy. Generally, the braze alloy has been provided only on the external side of the stamped component, i.e., the side opposite to the side in which the internal tubes were inserted. Typically, the cladding layers are an aluminum-silicon eutectic brazing alloy characterized by a melting point lower than the core aluminum alloy. Therefore, the clad layers of brazing alloy melt during the brazing operation and flow toward the desired joint regions and upon cooling solidify to form the brazements. The core aluminum alloy does not melt during the brazing operation. The aluminum alloy brazing stock material which has been routinely used to form these types of condensers consists of an aluminum alloy core layer that has been clad on at least one side by an aluminum-silicon brazing alloy.
Prior to brazing, the assembly is generally sprayed with or dipped into a flux mixture to enhance the brazeability of the brazing alloy. A conventional flux mixture consists of about 15 to about 25 volume percent flux solids suspended in water. The assembly is then dried to evaporate the water, leaving only the powdery flux solids on the external surface of the assembly. A satisfactory type of flux for use with these aluminum alloys has been potassium fluoaluminate complexes, as disclosed in U.S. Pat. Nos. 3,951,328 and 3,971,501 to Wallace et al and Cooke, respectively. However, because the flux is only introduced externally to the assembly (through dipping or spraying), the internal joints of the assembly, particularly the tube-to-header joints, do not directly benefit from the application of the flux, as evidenced by the high percentage, sometimes as high as about 10 to 30 percent, of defective leaky assemblies. These assemblies then require costly individual repair.
It would be desirable to provide a means for applying the flux mixture to the internal regions of the condenser so as to enhance the brazeability of the internal joints, particularly the tube-to-header joints. Until now, there has not been a satisfactory method for applying this flux to the internal joints. Generally, the reason for this has been that the flux mixture has been characterized by a sufficiently low viscosity such that the flux mixture is incapable of being consistently deposited at a predetermined region. Also, after evaporation of the aqueous vehicle, the flux is a particulate shape which does not adhere well to the surfaces of the condenser. Subsequent handling and assembly of the condenser would cause sufficient agitation to shake loose a portion of the flux particulates from the condenser surface.
Another shortcoming associated with the use of this conventional brazing approach is that during brazing, it is extremely important that the furnace atmosphere have a dewpoint of about -45.degree. F. or below and a free oxygen level of 100 parts per million (ppm) or less. A common approach has been to employ high purity cryogenic nitrogen. In a high dewpoint or high oxygen-containing atmosphere, a greater amount of oxidation of the aluminum occurs during the brazing cycle, thereby accordingly requiring greater quantities of flux. Therefore, with the conventional approach wherein the flux solids are suspended in water, all of the water must be removed prior to the brazing operation. This is difficult to consistently achieve in a production environment. In addition, entrapped moisture and oxygen inside the internal tanks and tubes of the condenser assembly also impede brazing, thereby requiring complete purging of the assembly just before the brazing operation, which is again costly and difficult to achieve.
Therefore, it would be advantageous to provide a method for brazing these types of condensers, wherein the flux can be pre-applied to specific regions during assembly so as to enhance the brazing of the internal joints, specifically between the tubes and headers. By improving the uniformity and consistency of the internal brazed joints, the number of defective leaky assemblies should be significantly reduced. In addition, it would also be advantageous if the flux of this invention did not require an aqueous-based vehicle, so as to minimize the amount of moisture surrounding the assembly during the brazing operation, thereby optimizing the efficiency of the brazing procedure.